1. Field of the Invention
This invention relates generally to semiconductor wafer polishing, and more particularly to drive mechanisms for gimbal projection systems in a wafer polishing environment.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform Chemical Mechanical Polishing (CMP) operations, including polishing, buffing and wafer cleaning. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. Patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases.
Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization. Further applications include planarization of dielectric films deposited prior to the metallization process, such as dielectrics used for shallow trench isolation or for poly-metal insulation.
In the CMP process, the gimbal point of a CMP substrate carrier is a critical element. The substrate carrier must align itself to the polish surface precisely to insure uniform, planar polishing results. Many CMP substrate carriers currently available yield wafers having anomalies in planarity. The vertical height of the pivot point above the polishing surface is also important, since the greater the height, the larger the moment that is induced about the pivot point during polishing. Two pervasive problems that exist in most CMP wafer polishing apparatuses are underpolishing of the center of the wafer, and the inability to adjust the control of wafer edge exclusion as process variables change.
For example, substrate carriers used on many available CMP machines experience a phenomenon known in the art as xe2x80x9cnose divingxe2x80x9d. During polishing, the head reacts to the polishing forces in a manner that creates a sizable moment, which is directly influenced by the height of the gimbal point, mentioned above. This moment causes a pressure differential along the direction of motion of the head. The result of the pressure differential is the formation of a standing wave of the chemical slurry that interfaces the wafer and the abrasive surface. This causes the edge of the wafer, which is at the leading edge of the substrate carrier, to become polished faster and to a greater degree than the center of the wafer.
The removal of material on the wafer is related to the chemical action of the slurry. As slurry is inducted between the wafer and the abrasive pad and reacts, the chemicals responsible for removal of the wafer material gradually become exhausted. Thus, the removal of wafer material further from the leading edge of the substrate carrier (i.e., the center of the wafer) experiences a diminished rate of chemical removal when compared with the chemical action at the leading edge of the substrate carrier (i.e., the edge of the wafer), due to the diminished activity of the chemicals in the slurry when it reaches the center of the wafer.
Apart from attempts to reshape the crown of the substrate carrier, other attempts have been made to improve the aforementioned problem concerning xe2x80x9cnose divingxe2x80x9d. In a prior art substrate carrier that gimbals through a single bearing at the top of the substrate carrier, sizable moments are generated because the effective gimbal point of the substrate carrier exists at a significant, non-zero distance from the surface of the polishing pad. Thus, the frictional forces, acting at the surface of the polishing pad, act through this distance to create the undesirable moments.
Further, the need for torsional drives that connect the gimbal to the driving spindle have proved unsuccessful in reducing the xe2x80x9cnose divingxe2x80x9d effect. In particular, using a single, or other direct drive means causes a force moment above the wafer that again causes xe2x80x9cnose diving.xe2x80x9d Moreover, drive pins are a source of backlash, since a pin needs to be free in a hole to allow pivoting.
In view of the foregoing, there is a need for a gimbal based torsion drive that is capable of driving a wafer without causing the wafer edges to dig into the on coming polishing pad. The drive should allow the wafer to be driven rotationally yet still pivot to allow for non-alignment of the rotational axis with the contact surface of the wafer being driven.
Broadly speaking, the present invention fills these needs by providing a drive mechanism that permits torque and axial force to be transmitted to a wafer being polished, not withstanding that the plane of the wafer might not be exactly perpendicular to the axis of rotation of the driving spindle. Thus, the drive mechanism allows the wafer to tilt about a gimbal point located on the surface of the wafer. In one embodiment, a projected gimbal point drive system is disclosed. The projected gimbal point drive system includes a spindle capable of applying a torque, and further having a concave spherical surface formed on its lower portion. Also included is a wafer carrier disposed partially within the lower portion of the spindle. The wafer carrier has a convex spherical surface formed on a surface opposite the concave spherical surface of the spindle. In addition, a drive cup is included that is disposed between the spindle and the wafer carrier. The drive cup has a concave inner surface and a convex outer surface, and allows the wafer carrier to be tilted about a predefined gimbal point. The gimbal point can be located on an interface between a polishing pad and a surface of a wafer held by the wafer carrier. Further, the gimbal point can be intentionally located above (xe2x80x9cnose divingxe2x80x9d) or below (skiingxe2x80x9d) the interface between a polishing pad and a surface of a wafer held by the wafer carrier if desired.
In another embodiment, a projected gimbal point drive cup is disclosed. The projected gimbal point drive cup includes a first set of elongated slots located in a convex outer surface of the drive cup, and a second set of elongated slots located in a concave inner surface of the drive cup. The drive cup allows a wafer carrier to be tilted about a predefined gimbal point. A first set of drive keys extending out of a concave spherical surface of a spindle can be used to extend into the first set of slots in the drive cup. Similarly, a second set of drive keys extending out of a convex spherical surface of the wafer carrier can extend into the second set of slots of the drive cup. Optionally, the first set of slots can comprise two elongated slots, which are separated by about 180 degrees around the circumference of the drive cup. Similarly, the second set of slots can comprise two elongated slots, which also are separated by about 180 degrees around the circumference of the drive cup. Further, the first set of slots can be located about ninety degrees around an axis of symmetry of the drive cup from the second set of elongated slots.
A method for driving a projected gimbal point system is disclosed in a further embodiment of the present invention. A spindle is provided that is capable of apply a torque. The spindle includes a concave spherical surface formed on a lower portion of the spindle. Also, a wafer carrier is disposed partially within the lower portion of the spindle. The wafer carrier includes a convex spherical surface formed on a surface opposite the concave spherical surface of the spindle. The spindle is then coupled to the wafer carrier using a drive cup disposed between the spindle and the wafer carrier. As above, the drive cup includes a concave inner surface and a convex outer surface, and allows the wafer carrier to be tilted about a predefined gimbal point. The gimbal point can be located on an interface between a polishing pad and a surface of a wafer held by the wafer carrier. Optionally, the gimbal point can be intentionally located above or below the interface between a polishing pad and a surface of the wafer held by the wafer carrier as desired.
Advantageously, the embodiments of the present invention can be configured such that the spherical shape and concentricity of the surface of the lower part of the drive spindle and surface of the wafer carrier assure that the wafer can tilt only about an axis that lies in the plane of the wafer-pad interface. If the axis about which the wafer tilts lies above or below the wafer-pad interface, forces are generated that push one sector of the wafer into the polishing pad more strongly than the diametrically opposite sector of the wafer is pushed, resulting in undesirable effects. The embodiments of the present invention allow these forces to be reduced, eliminated, or employed deliberately in a controlled manner to produce a desired result. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.